The invention relates generally to the field of computer data storage, and in particular, to a multi-tier data storage system and methods for handling data in the multi-tier data storage system.
The rapid rate of innovation in processor engineering has resulted in an impressive leap in performance from one computer generation to the next. While the processing capability of the computer has increased tremendously, the input/output (I/O) speed of secondary storage devices such as disk drives has not kept pace. Whereas the processing performance is largely related to the speed of its electronic components, disk drive I/O performance is dominated by the time it takes for the mechanical parts of the disk drives to move to the location where the data is stored, known as a seek and rotational times. On the average, the seek or rotational time for random accesses to disk drives is an order of magnitude longer than the data transfer time of the data between the processor and the disk drive. Thus, a throughput imbalance exists between the processor and the disk system.
To minimize this imbalance, conventional disk systems typically use a disk cache to buffer the data transfer between the host processor and the disk drive. The disk cache reduces the number of actual disk I/O transfers since there is a high probability that the data accessed is already in the faster disk cache. The operating principle of the disk cache is the same as that of a central processing unit (CPU) cache. The first time a program or data location is addressed, it must be accessed from the lower-speed disk memory. Subsequent accesses to the same code or data are then done via the faster cache memory, thereby minimizing its access time and enhancing overall system performance. The access time of a magnetic disk unit is normally about 10 to 20 ms, while the access time of the disk cache is about one to three milliseconds. Hence, the overall I/O performance is improved because the disk cache increases the ratio of relatively fast cache memory accesses to the relatively slow disk I/O access. The caching principle can be further extended so that faster disks act as caches for slower data storage devices. For instance, a magnetic data storage device can cache data from a slower device such as a compact disk (CD) drive, a digital video disk (DVD) drive, or an archival tape/optical disk back-up system.
Many applications require the architecture of the data storage system needs to provide varying degrees of high performance, reliability and cost-effectiveness. For instance, media server applications need to support widespread availability of interactive multimedia services such as for viewing and retrieving high-resolution digital photographic images. Other applications include video-on-demand (VOD), teleshopping, digital video broadcasting and distance learning. Typically, a media server retrieves digital multimedia bit streams from storage devices and delivers the streams to clients at an appropriate delivery rate. The multimedia bit streams represent video, audio and other types of data, and each stream may be delivered subject to quality-of-service (QOS) constraints such as average bit rate or maximum delay jitter. An important performance criterion for a media server and its corresponding multimedia delivery system is the maximum number of multimedia streams, and thus the number of clients, that can be simultaneously supported. In addition to being performance driven, these multimedia servers require their data storage systems to be able to store, retrieve and archive terabytes of data across diverse and geographically distributed networks. Further, to be commercially successful, these requirements should be provided as cost-effectively as possible.